The use of seals for sealing gaps between a rotating and a non-rotating element, such as those found in gas turbine engines, in order to prevent fluids from leaking around rotating equipment is well known in the art. Conventional dynamic seals used for rotating equipment such as gas or steam turbine engines, compressors and the like, are traditionally designed as either non-contacting seals having a gap between the seal and the rotating surface, for example a labyrinth seal, or as contacting seals without a gap for example, brush seals, segmented carbon seals, and the like. Contacting seals are designed to minimize leakage at the cost of friction and wear of seal faces. On the other hand, non-contacting seals are designed to operate with some face separation to reduce frictional heat generation and wear at the cost of some leakage. Because of the size of the gaps typically created, non-contacting labyrinth seals have been found to have high levels of leakage during use. However, an advantage of such labyrinth seals is that they are traditionally made from metallic structures and, hence, can be used at high temperatures and for high surface speed applications. Generally, such seals are designed with a gap greater than the normal rotor growth and excursions to avoid rotor contact. In unusual cases when the rotor does contact the seal, the seal gap further enlarges, thereby, degrading seal performance and equipment efficiency.
Other types of non-contacting seals, such as non-contacting hydrodynamic and hydrostatic seals used in turbomachinery, particularly in gas compressors, are also known in the art. Unlike labyrinth seals, non-contacting hydrodynamic seals run on very thin fluid or gas films, typically in the range of about 50–300 μinch, as compared to labyrinth radial clearances which are typically in the range of 0.02–0.1 inch. Non-contacting circumferential seals have also posed design challenges. Some of the design requirements of non-contacting, particularly circumferential seals include: 1.) the ability of the seal inner diameter (ID) to conform to the variation of the rotor outer diameter (OD); 2.) the provision of hydrodynamic and/or hydrostatic grooves at the seal ID to provide seal lift-off at relatively low r.p.m to minimize groove wear; 3.) the balancing of seal radial compliance to conform to the rotor OD and sufficient axial stiffness to withstand the differential pressure between the high and low pressure sides; 4.) the provision of fluid or gas film stiffness sufficiently higher than the radial structural stiffness of the seal so as to provide non-contact operation at high speeds even during transient rotor excursions; and 5.) the use of high temperature/high strength superalloy materials for high temperature operation while also providing for shallow hydrodynamic/hydrostatic grooves.
The face seals of non-contacting seals can be polished extremely flat so that mating surfaces are parallel and can operate with very small gap clearance in the range of about 50–300 μinch, to provide stable non-contacting film riding conditions, as known in the art. In such seals, the dimensions required have been found to be critical and the seal face materials should not distort over a wide range of temperature and pressure. Conventional non-contacting seals may include very shallow (100–300 μinch) hydrodynamic and/or hydrostatic grooves such as spiral grooves, Raleigh pads or radial grooves, as is also known in the art. As the rotor speed and system pressure increase, fluid or gas pressure within these shallow grooves build up which can cause the rotor to unacceptably separate from the seal interface. The seal opening force generated by these shallow grooves drops off rapidly as the gap between the sealing surfaces increases. For circumferential seals, the sealing interface is circular around the circumference of the rotor. Lastly, the clearance of the sealing interface cannot be controlled as precisely as the face seals because of a number of factors, including rotor non-circularity, run-out, radial excursion, centrifugal growth and the like.
Segmented carbon seals are contacting seals that typically have a much lower leakage than labyrinth seals, as they tend only to make light contact with the rotating surface. However, the application temperature and surface speed of segmented carbon seals are both limited. For example, at temperatures above about 1000° F. and surface speeds above about 300 ft/sec., segmented carbon seals are generally not used due to performance limitations. Brush seals are traditionally made of high temperature superalloy bristles and, hence, their application temperatures and surface speeds are greater than those of segmented carbon seals. Typically, brush seals are used in applications up to about 1200° to 1400° F. and speeds up to about 1000–1200 ft/sec. For example, in gas turbine engines brush seals are often utilized to minimize leakage of fluids at circumferential gaps, such as between a machine housing and a rotor, around a rotary shaft of the engine, and between two spaces having different fluid pressure within the engine. In turbine engines, the fluid pressure within the system, (which may be either liquid or gas) is greater than the discharge pressure (the pressure outside the area of the engine housing, toward which the fluid will tend to leak), thus creating a pressure differential in the system. As used herein, the system pressure side of is referred to as the high pressure side, while the discharge pressure side is referred to as the low pressure side.
Both carbon segmented seals and brush seals wear with time, and as they wear their performance degrades, as known to those of skill in the art. In addition, metallic brush seals are not generally used at surface speeds greater than about 1200 ft/sec. because of the potential for bristle tip melting. Surface speeds in the vicinity of a gas-path can generally be greater than about 1200 ft/sec and, thus, brush seals are typically not suitable for such applications.
In order to enhance efficiency of rotating equipment, both the application temperature and surface speed are constantly being increased. Currently, the only known sealing system available for these extremely high temperature/high surface speed applications is the larger-gap labyrinth seal.
Therefore, there is a need for advanced high temperature seals which are capable of running with a smaller gap or clearance compared to labyrinth seals for high temperature and speed applications in order to enhance equipment efficiency.